Indium solder has been widely used in the semiconductor art for mounting semiconductor devices onto heatsinks. The low temperature (m.p. 156.degree. C.) bond formed when soldering with indium is relatively soft causing very little stress to the mounted device. However Indium and other low temperature solders have the disadvantage that the limits of operating temperatures of the mounted devices will also be low and this is not acceptable for many current devices. These lower melting point solders are known to diffuse and creep at temperatures considerably below their melting points, weakening the bond strength and many times causing contamination to the device. Further, many solders, including indium, typically require the use of a flux for proper wetting, but the flux is a known source of contamination and is also thought to be responsible for voids in the bond which reduce the bond strength and can adversely affect the thermal and electrical properties of the bond.
Gold-tin eutectic alloy solders are also known for bonding semiconductor devices to heatsinks. Although gold-tin solders have the disadvantage that some stress may be introduced to the device during soldering, the advantages of gold-tin solders include higher operating temperatures, less contamination, better thermal and electrical conductivity and increased bond strength. A widely used and very desirable alloy from this group comprises 80 percent by weight of gold and 20 percent by weight of tin. This alloy has a melting point of 280.degree. C.
One semiconductor device which would benefit considerably from the advantages of a 80Au20Sn bond is the InGaAs long wavelength photodetector. Most current applications for this and other semiconductor devices require a device-to-heatsink bond of the quality offered by a gold-tin eutectic alloy.
Unfortunately, when InGaAs dectetors are heated to 280.degree. C. and above, as is necessary in bonding the device to the heatsink using 80Au20Sn, the detectors are adversely affected. In has been found that the leakage current which corresponds to the background noise of the device and is therefore a way of looking at the device integrity, increases to unsatisfactory levels when the device is exposed to temperatures of 250.degree. C. and above. Thus, 250.degree. C. is felt to be a critical temperature for InGaAs photodetectors.
The need to bond InGaAs detectors having a critical temperature of about 250.degree. C. using a 280.degree. C. m.p. solder, has led to attempts at employing thermocompression techniques. As is known in thermocompression bonding art, the parameters of temperature, pressure and time can be varied within certain limits to lower the bonding temperature required in a given process. For example, Dale in U.S. Pat. No. 3,883,946 disclosed bonding using temperatures 15.degree. to 50.degree. C. below the melting points of various solders by applying pressures between about 140 to 700 kg/cm.sup.2 to the devices during bonding. Gold-tin, known to be a "hard" solder, necessitates higher pressures than those disclosed by Dale to lower the bonding temperature below the 250.degree. C. critical point.
It has been found however that the InGaAs detectors cannot withstand pressures in this range for several reasons. First, the InGaAs device has a certain amount of inherent internal stress which is a function of the crystalline structure of the device. Further, the 80Au20Sn solder induces more stress after bonding because of the variations in the coefficient of expansion between the solder and the device. Finally, the addition of pressures in the range of 140 to 700 kg/cm.sup.2 during bonding imparts amounts of stress which culminate in unacceptably short device lifetimes and unstable device operation.
For sensitive devices such as InGaAs photodetectors it would be desirable, therefore, to have a flux-free process to bond them to a heatsink using a higher temperature, corrosion resistant solder such as gold-tin, but without adversly affecting the detectors via high temperatures and/or pressures.